Triduum is a fancy church word that means three whole days that we celebrate every year right before Easter.
The first day is called Holy Thursday, and we remember when Jesus had a special meal with his friends (called the Last Supper) and he gave them bread and wine to eat and drink, telling them that it was his body and blood.
The second day is called Good Friday, and we remember when Jesus died on the cross. It's a sad day, but it reminds us of how much Jesus loved us because he was willing to die for us.
The third day is called Holy Saturday, and it's a day of waiting and preparing for Easter. We remember that after Jesus died, he was buried in a tomb, and we wait to celebrate when he rose from the dead on Easter Sunday.
So really, triduum is just a big celebration that helps us remember how much Jesus loved us and what he did for us.